This invention relates to a radio frequency coupling device including a ferrimagnetic coupling element.
A particular, though not exclusive, application of the invention is to microwave tunable filters and limiters. Tunable microwave filters have been produced in which radio frequency power is coupled from one transmission line to another through a magnetised body of ferrimagnetic material commonly called a resonator. When used in a filter, the form of coupling through the body is by uniform spin precession in the ferrimagnetic material about an applied direct magnetic field. The response of the resonator is a function of the applied field as well as the geometry of the body and coupling is obtained in a band centered on the resonant frequency. Thus this form of coupling is not only frequency selective but is tunable by changing the applied bias field.
It is also known that the ferrimagnetic resonator coupling falls above certain input power level at which uniform spin precession begins to breakdown. It is, therefore, possible to make limiters based on this effect.
Initially resonators of the above kind were ferrimagnetic spheres, the ferrimagnetic material being Yttrium Iron Garnet (YIG). Subsequently YIG discs were used and more recently the use of epitaxially grown YIG films has been investigated, the film being formed to a disc shape.